1. Field of the Invention
The present invention relates to a fuel cell formed by sandwiching a membrane electrode assembly between a pair of separators. The membrane electrode assembly includes an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode. Further, the present invention relates to a method of producing the fuel cell.
2. Description of the Related Art
A solid polymer electrolyte fuel cell as one type of fuel cell employs a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is a polymer ion exchange membrane. In the solid polymer electrolyte fuel cell of this type, an anode and a cathode each having an electrolyte catalyst layer and a gas diffusion layer are formed on surfaces of the solid polymer electrolyte membrane to form a membrane electrode assembly. The membrane electrode assembly is sandwiched between a pair of separators (bipolar plates) to form a unit cell for generating electricity. In use, typically, a predetermined number of unit cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas flow field for supplying a fuel gas (e.g., hydrogen) is formed on a separator surface facing the anode, and an oxygen-containing gas flow field for supplying an oxygen-containing gas (e.g., air) is formed on a separator surface facing the cathode. Hereinafter, the fuel gas and the oxygen-containing gas may be referred to as a reactant gas. Further, a coolant flow field is formed between the separators for supplying a coolant along the surfaces of the separators for each unit cell, or for every predetermined number of unit cells.
As is known in the art, in operating the fuel cell, the reactant gases and the coolant flow into the flow fields of the fuel cell stack. For this purpose, seals are formed at the edges of both surfaces the separator and near the flow fields of the separator. The seals prevent leakage of the reactant gases and the coolant to the outside of the fuel cell stack.
Additionally, the seals function as insulating coating for preventing corrosion of the separator. That is, the seal is made of insulating material.
As the material for the seal, silicone rubber is used widely. The seal of the silicone rubber of this type may be provided by forming a seal member beforehand, and then, attaching the seal member to the separator. Preferably, as disclosed in Japanese Laid-Open Patent Publication No. 11-309746, the seal is formed by injection molding. In this manner, seals can be provided accurately at predetermined positions of the separator. Stated otherwise, the seals can be positioned with a high degree of accuracy advantageously.
At the time of carrying out the injection molding, as shown in FIG. 8, firstly, a separator 1 is provided at a predetermined position of one molding die 2. In this state, the remaining molding die 3 moves closer to the molding die 2, and thus, so called die locking is carried out. As a result, a cavity 4 is formed. Then, a rubber 5 having fluidity (e.g., melted or liquid substance) is supplied into the cavity 4. When the rubber 5 is hardened, seals are formed on both surfaces at the edges of the separator 1.
In this case, when the separator 1 is thick due to dimensional variation, since the separator 1 blocks the molding die 3, die locking may not be carried out sufficiently. Under the circumstances, as shown in FIG. 8, a very small gap 6 is formed between the molding dies 2, 3. In this state, when the rubber 5 is supplied into the cavity 4, the rubber 5 is filled in the gap 6. The rubber 5 hardened in the gap 6 forms so called burrs 7. When die opening is carried out, the remaining burrs 7 are present in the molding die 2 or the molding die 3. As can be seen from the above, in the case of forming the rubber seals by injection molding, the molding dies 2, 3 may be tainted by the remaining burrs 7 disadvantageously. For this reason, operation of the injection molding machine needs to be stopped each time the stains in the die due to the burrs 7 are removed. Accordingly, the production efficiency is lowered disadvantageously.
In the example shown in FIG. 8, the total amount of the rubber corresponding to the volume of the cavity 4 is supplied by one time injection to form the seals on both surfaces of the separator 1. In this case, the separator 1 may be deformed by the pressure of the rubber having fluidity. In this regard, in another known method, after a seal is formed on one surface of the separator 1 by injection molding, a seal is formed on the other surface of the separator 1 by the second injection molding. However, in this method, though it is possible to suppress deformation of the separator 1, since injection molding needs to be carried out twice, the production efficiency is lowered disadvantageously.
Additionally, the rubber such as a silicone rubber used for the seal is expensive. Therefore, it is desirable to reduce the amount of the rubber used for the seal as much as possible.